inches (in) 
millimeters (mm) 

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1 in = 25.4 mm  1 mm = 0.03937008 in 
1 
* 
2.54 1 
* 
10 mm 1 
= 
25.4 mm 
1 
* 
25.4 mm 1 
= 
25.4 mm 
Inches  1 
Feet  0.08333333333 
Yards  0.02777777778 
Miles  0.00001578282828 
Nautical Miles  0.00001371490281 
Lightyears  2.6847757e18 
Centimeters  2.54 
Meters  0.0254 
Kilometers  0.0000254 
We know 1 Inch is equal to 25.4 mm and Inch is a larger unit and mm smaller. So whenever we go to convert mm to inches, we have to remember that if we want to convert from small units to large units, we have to divide. And if you want large unit to small unit, we have to do multiple.
For example, we divided 15 by 25.4 to convert 15mm to inches. Since mm is a small unit and Inches large. And we know 1in = 25.4mm.
So to convert 15 Inches to mm, we need to multiply 25.4 by 15. Since we are going to convert from Inches to mm. We need to remember all the time to convert from large units to small units to multiply.
Convert 15 mm to inches:
d_{(″)} = 15mm / 25.4 = 0.5905″
Note: Fractions are rounded to the nearest 8th fraction. Values are rounded to 4 significant figures.
Millimeters  Inches (decimal)  Inches (fraction) 

1 mm  0.03937"  3/64" 
2 mm  0.07874"  5/64" 
3 mm  0.11811"  1/8" 
4 mm  0.15748"  5/32" 
5 mm  0.19685"  13/64" 
6 mm  0.23622"  15/64" 
7 mm  0.275591"  9/32" 
8 mm  0.314961"  5/16" 
9 mm  0.354331"  23/64" 
10 mm  0.393701"  25/64" 
11 mm  0.433071"  7/16" 
12 mm  0.472441"  15/32" 
13 mm  0.511811"  33/64" 
14 mm  0.551181"  35/64" 
15 mm  0.590551"  19/32" 
16 mm  0.629921"  5/8" 
17 mm  0.669291"  43/64" 
18 mm  0.708661"  45/64" 
19 mm  0.748031"  3/4" 
20 mm  0.787402"  25/32" 
21 mm  0.826772"  53/64" 
22 mm  0.866142"  55/64" 
23 mm  0.905512"  29/32" 
24 mm  0.944882"  15/16" 
25 mm  0.984252"  63/64" 
26 mm  1.0236"  1 1/32" 
27 mm  1.063"  1 1/16" 
28 mm  1.1024"  1 7/64" 
29 mm  1.1417"  1 9/64" 
30 mm  1.1811"  1 3/16" 
31 mm  1.2205"  1 7/32" 
32 mm  1.2598"  1 17/64" 
33 mm  1.2992"  1 19/64" 
34 mm  1.3386"  1 11/32" 
35 mm  1.378"  1 3/8" 
36 mm  1.4173"  1 27/64" 
37 mm  1.4567"  1 29/64" 
38 mm  1.4961"  1 1/2" 
39 mm  1.5354"  1 17/32" 
40 mm  1.5748"  1 37/64" 